Thin film deposition process

ABSTRACT

A thin film deposition process for depositing a metal oxide layer onto a substrate body, the process comprising the steps of taking a metal organic compound in vapor form, the compound being capable of absorbing light in the wavelength range of 240 to 400 nanometers, heating the substrate to a temperature below the pyrolytic decomposition temperature of the compound in the presence of the vapor, directing light towards the substrate to cause a photochemical fragmentation of said vapor molecules, the reaction resulting in a deposition of the required oxide on said substrate. 
     The metal organic compound may be a metal alkoxy substituted beta-diketonate such as aluminium diisopropoxide acetyl acetonate.

This invention relates to a thin film deposition process. It relatesparticularly to such a process which can be used for the thin filmdeposition of various dielectric layers for electronic applications.

The present invention was devised in an attempt to provide a lowtemperature technique that could be used for the area selectivedeposition of thin film dielectric materials based on oxides ornitrides. The method chosen was to irradiate a substrate body withultraviolet light whilst passing a vapour of an organometallic compoundover the substrate, such that the organometallic compound would absorbthe incident light and undergo photofragmentation to form a product thatwould deposit on the substrate surface.

Such a process has been used already for the deposition of a wide rangeof metal containing compounds and it has often been necessary to use alight source producing very short wavelengths (about 200 nanometers) ofradiation in order to achieve the required level of photo absorption inthe available precursor materials. Such a light source is often notreadily available and it is relatively costly one to provide. It mayalso require expensive optics or the need to eliminate the presence ofoxygen from the light path.

It would therefore be more convenient to use a different type ofradiation source which is more freely available and which produces alonger wavelength (for example, 250 to 400 nanometers) of radiation.However, it was found that the precursors which are commonly used formetal oxide deposition, which are alkoxide compounds, do notsignificantly absorb light at wavelengths greater than 200 nanometers.

An object of the present invention, therefore, was to provide precursorcompounds for metal oxide deposition which compounds would havesatisfactory absorption bands for light of the longer wavelength.

According to the invention, there is provided a metal oxide depositionprocess, in which an organometallic precursor compound is a metal alkoxysubstituted beta-diketonate. In the beta-diketonate, thealpha-substitutions to the oxygen atoms may be individually selectedfrom alkyl, fluorinated alkyl, alkoxy or aryl and the beta-substitutionto the oxygen atoms is selected from hydrogen, halogen or lower alkyl.

Preferably, the compound is capable of absorbing light in the wavelengthrange of 240 and 400 nanometers. A more preferred range is between 250and 300 nanometers.

The diketonate may be aluminium diisopropoxide acetyl acetonate.

According to a further aspect, there is provided a thin film depositionprocess in which a dielectric layer may be laid down on a substrate bodysurface at a comparatively low temperature, the process comprising thesteps of taking a metal organic compound in vapour form the compoundbeing capable of absorbing light in the wavelength range of 240 to 400nanometers, heating the substrate body to a temperature below thepyrolytic decomposition temperature of the compound in the presence ofthe vapour, directing light of a predetermined wavelength towards thesubstrate to cause a photochemical fragmentation of said vapourmolecules, the reaction resulting in a deposition of the required oxideon said substrate.

The metal organic compound may be a metal alkoxy substitutedbeta-diketonate such as aluminium diisopropoxide acetyl acetonate.

The invention further comprises an electronic device including a thinoxide layer deposited by the abovementioned process.

By way of example, a particular embodiment of the invention will now bedescribed with reference to the accompanying drawing, in which:

FIG. 1 shows the general formula and structure of one type oforganoaluminium compound of the invention,

FIG. 2 shows apparatus for carrying out the process of the invention,and,

FIG. 3 shows a substrate body supporting a deposited metal oxide layer.

In the organoaluminium compound of FIG. 1, R is preferably hydrogen butit may alternatively be halogen or a low molecular weight alkyl. Theradicals R₁ and R₂ may be individually selected from the group of alkyl,alkoxy, aryl, or fluorinated alkyl.

The preferred substitutions to be incorporated depend on the intendedlight source to be used. For illumination by an unfiltered mercury lampthe preferred compound would be one where R═H and R₁ ═R₂ ═CH₃ since thiscompound absorbs light strongly at wavelengths between 240 and 300nanometers and has high volatility.

The wavelength at which maximum absorption occurs is 380 nanometers andthis compound is found to be one of the most volatile of this group ofprecursors. Where a monochromatic illumination source, such as the 257nanometer line from a frequency doubled argon ion laser is used then theabsorption peak may be localised to coincide with that line. Thus, for257 nanometers, the preferred compound would be R═H and R₁ ═R₂ ═OC₂ H₅,since this compound has a photoabsorption peak at 257 nanometers andpossesses a reasonable volatility. In alternative compound, R₁ ═OCH₃, R₂═CH₃ and this has a photoabsorption peak at 261 nanometers in thevapour. A general method for the synthesis of these various metal oxideprecursors with R═H and R₁ ═R₂ ═CH₃ will be described later.

As depicted in FIG. 2, the deposition apparatus comprises a stainlesssteel deposition chamber 1 provided with an ultraviolet transmissionwindow 2 of quartz. The chamber 1 contains a table 3 which supports asteel heater block 4 having electrical supply leads 6. The heater block4 is in contact with a substrate body 7 upon an upper surface of whichthe required metal oxide layer is to be deposited. Outside the window 2is located alight source 5.

A carrier gas supply for the chamber 1 is provided by an argon gassupply source 8 which delivers a gas stream passing through a needlevalve 9 connected to a gas flow meter 11 and then through a bubbler tube12 surrounded by a heating oil jacket 13. The bubbler tube 12 is filledwith a volatile liquid precursor of the metal compound required to bedeposited in the chamber 1.

The gas flow outlet from the bubbler tube 12 passes through furthertubing surrounded by a heating jacket 14 and then enters the chamber 1.

A gas outlet from the chamber 1 passes through a length of flexiblevacuum tubing 16 which conducts exhausted gas between the walls of aglass dewar flask 17 and thence to a vacuum system 18. The dewar flask17 is filled with a liquid nitrogen coolant 19.

Use of this apparatus to deposit a metal oxide film will be explainedshortly, after the synthesis of one precursor compound, diisopropoxyaluminium acetylacetonate has been described.

In order to prepare the precursor, pentan-2,4-dione (6.56 g) was addedto aluminium isopropoxide (13.4 g) in benzene (120 milliliters) and themixture was heated under reflux conditions for two hours. The resultingsolution was slowly distilled until the vapour temperature rose to 80°C. The remaining benzene was rapidly distilled off. The oil whichresulted was distilled under vacuum to yield diisopropoxy aluminiumacetylacetonate as a pale yellow oil that became crystallised upon beingleft to stand for several days.

In order to deposit the required metal oxide, the precursor compoundjust described was loaded into the bubbler tube 12 and this was heatedby the oil jacket 13 to 125° C. The deposition chamber 1 and bubblertube 12 were held under reduced pressure by means of the vacuum system18 which was constituted by a large capacity rotary pump backing an oildiffusion pump. A carrier gas was bubbled through the aluminium compoundand argon was used to flush the chamber window to give a chamberpressure of about 0.5 mbar. Volatile reactants and reaction productswere condensed in the liquid nitrogen cold trap formed by the dewarflask 17 before the non-condensable vapours entered the vacuum system18.

The light source 5 was formed by a mercury arc lamp of one kilowattpower. The light was focussed through the window 2 onto the substrate 7which was maintained at a temperature of 200° C. by the heater 4.

Use of these process conditions resulted in the formation of analuminium oxide film 21 (FIG. 3) on the substrate 7 surface. Nodeposition was found to occur in the absence of illumination, althoughdeposition could be made to take place by the step of raising thesubstrate temperature to 500° C.

The advantages of this use of a metal diketonate for oxide depositioninclude the feature that the photoabsorption of the metal oxideprecursors may be matched, by appropriate substitution, to theirradiation source being used. A range of possible compounds which havebeen synthesized in the laboratory is given in the following Table withsome physical characteristics.

                  TABLE                                                           ______________________________________                                                                     Vapour                                                                        Pressure                                         Com-  Substituents           at     Wavelength                                pound for diisopro-                                                                             Physical   120° C.                                                                       of Absorption                             Num-  poxy aluminium                                                                            Charac-    (milli-                                                                              Maximum                                   ber   diketonate  teristics  bars)  (nanometers)                              ______________________________________                                        1     R = H       Extremely  0.53   257/256                                         R.sub.1 = OC.sub.2 H.sub.5                                                                viscous                                                           R.sub.2 = OC.sub.2 H.sub.5                                                                colourless                                                                    liquid (glass)                                                                at room                                                                       temperature                                                 2     R = H       Colourless 1.80   267/266                                         R.sub.1 = OC.sub.2 H.sub.5                                                                viscous liquid                                                    R.sub.2 = CH.sub.3                                                                        at room                                                                       temperature                                                 3     R = H       Yellow     1.30   269/261                                         R.sub.1 = OCH.sub.3                                                                       crystalline                                                       R.sub.2 = CH.sub.3                                                                        solid at room                                                                 temperature.                                                                  MP about                                                                      60° C.                                               4     R = H       Yellow     3.00   287/284                                         R.sub.1 = CH.sub.3                                                                        crystalline                                                       R.sub.2 = CH.sub.3                                                                        solid at room                                                                 temperature.                                                                  MP about                                                                      105°  C.                                             5     R = H       Colourless  --    293/--                                          R.sub.1 = C(CH.sub.3).sub.3                                                               liquid,                                                           R.sub.2 = C(CH.sub.3).sub.3                                                               thermally                                                                     unstable                                                    6     R = H       Brown liquid,                                                                             --    297/--                                          R.sub.1 = C(CH.sub.3).sub.3                                                               thermally                                                         R.sub.2 = CF.sub.3                                                                        unstable                                                    ______________________________________                                    

For the values of wavelength Absorption Maximum which are given, thefirst figure gives that for the compound in chloroform solution and thesecond figure gives the maximum value for the compound vapour.

For deposition of the required metal oxide, Compound 1 was illuminatedwith the frequency doubled argon ion laser light (257 nanometers) whilstCompounds 3 and 4 were illuminated with a xenon mercury lamp for the 250to 300 nanometer region.

The precursors and process of the invention thus can permit thedeposition of oxides at temperatures which are 200° to 300° C. lowerthan deposition using non-photochemical techniques. The preferredprecursors allow the use of less costly light sources at wavelengthsthat do not require expensive optics or the removal of oxygen from thelight path.

The foregoing description of embodiments of the invention has been givenby way of example only and a number of modifications may be made withoutdeparting from the scope of the invention as defined in the appendedclaims. For instance, it is not essential that the technique should beapplied only to aluminium compounds and other metals such as titanium,zirconium, strontium may also be suitable. In addition, the substitutionof more than a single diketone for the alkoxy groups is also feasible.An area selective deposition of the oxide layer could be effected, forexample, by suitable shaping the light beam which is directed throughthe window 2 or by moving a laser beam in order to `write` in the oxidearea that is required.

The process is believed also to be suitable for depositing alternativedielectric materials such as nitrides.

We claim:
 1. A thin film deposition process in which a dielectric layermay be laid down on a substrate body surface at a comparatively lowtemperature, the process comprising the steps of taking a metal alkoxysubstituted beta-diketonate in vapour form, the metal alkoxy substitutedbeta-diketone being capable of absorbing light in the wavelength rangeof 240 to 400 nanometers, heating the substrate body to a temperaturebelow the pyrolytic decomposition temperature of the metal alkoxysubstituted beta-diketonate in the presence of the vapour, directinglight of a predetermined wavelength in the range of 240 to 400nanometers towards the substrate to cause a photochemical fragmentationof said vapour molecules, the reaction resulting in a deposition of thecorresponding metal oxide on said substrate.
 2. A process as claimed inclaim 1, wherein in the metal alkoxy substituted beta-diketonate thealpha-substitutions to the oxygen atoms are individually selected fromthe group consisting of alkyl, fluorinated alkyl, alkoxy and arylgroups, and the beta-substitution to the oxygen atoms is selected fromthe group consisting of hydrogen, halogen and lower alkyl.
 3. A processas claimed in claim 1, in which the wavelength range is 250 to 300nanometers.
 4. A process as claimed in claim 1, in which the metalalkoxy substituted beta-diketonate is aluminium diisopropoxide acetylacetonate.